Canker sores tend to be inside the mouth, cold sores outside. There is a
genetic component to canker sores, which usually onset early in life.
Canker sores (according to the National Institute of Dental Health - or
some title close to that) are not contagious. Links to nutritional
deficits, immune malfunctions and allergies are all suspected to play a
role. It is thought that all canker sores, even in a single individual,
do not all have the same cause.
(I did an article on dental and oral health recently, and so happen to
have read up on the subject...)
E.